From the instructions for a Form 433-D - https://www.irs.gov/pub/irs-pdf/f433d.pdf

Review the terms of this agreement. When you’ve completed this agreement form, please sign and date it. Then, return Part 1 to IRS at the address on the letter that came with it or the address shown in the “For assistance” box on the front of the form.

If you are requesting an initial installment agreement, I suggest you use form 9465 and the mailing address for form 9465 is identified on page 2 of the instructions for form 9465.  You can get a copy of form 9465 and the instructions for form 9465 at www.irs.gov.  When the IRS screen opens, select "Forms and Pubs" in upper left corner then enter 9465 in the search box on the next screen.  The search will yield data for form 9465 which you can download and actually complete on the screen once you have downloaded the form.  I suggest you DO NOT fill in the form while in the IRS web site since the information is not protected.  Fill the form once you have it downloaded to your computer.